The invention relates to a method of manufacturing flakes from a magnetically hard material by spraying a molten alloy which comprises at least a rare earth metal, at least a transition metal from the group consisting of iron and cobalt, and boron on a rapidly moving cooling surface, a magnetically hard finely crystalline phase being formed in the solidified alloy.
Flakes are to be understood to mean in this connection relatively short pieces of a metal foil of a small thickness having a length which may be a few times larger than their width.
The invention also relates to flakes of a magnetically hard material of the said composition and to magnets manufactured from flakes according to the invention.
A method of manufacturing a magnetically hard material in the form of a ribbon is known from European Patent Application EP-A 108 474. The moving cooling surface in the known method is formed by the circumferential surface of a disc of a material having a high thermal conductivity, for example copper, covered with chromium.
It is stated on pages 47 and 48 of this European Patent Application that the optimum magnetic domain dimension is approximately 40 to 50 nanometers (0.04-0.05 .mu.m). Alloys with crystallites having dimensions between approximately 20 and 400 nanometers show permanent magnetic properties. The ribbons are magnetically isotropic. This appears from the fact that the ribbons show the same magnetizability in all directions. It is suggested in the European Patent Application that a preferred orientation of the crystallites might result in improved magnetic properties. However, this European Patent Application does not give any indication as to how such a preferred orientation could be realised.
From the published Japanese Patent Application (Kokai) No. 61-10208 a method of manufacturing permanent magnets from the said metals is known, in which in a first step thin ribbons are manufactured by spraying molten metal on a moving cooling surface. In a second step, involving the application of heat, a laminated body is manufactured from the ribbon obtained in the first step.
In the first step of this method thin ribbons are obtained by employing a certain method of cooling, in which crystals are present whose c-axis is perpendicular to the surface of the ribbon.
With respect to the method of cooling it is only indicated that the rate of movement of the cooling surface must be from 3-20 m/s. From the exemplary embodiments it appears that coherent ribbons having a thickness of 100 .mu.m are manufactured which may be divided into parts having a length of 10 mm each.
In the second step of this method a permanent magnet having suitable properties is obtained by heating (600.degree.-1100.degree. C.) under pressure (2 tons/cm.sup.2). This heating under pressure is essential for obtaining a high-energy product (BH max).
In the investigations leading to the invention it has been found that when in the known method ribbons of a greater length and having a thickness of 100 .mu.m are manufactured, orientation of the c-axis perpendicularly to the surface of the ribbon occurs, mainly in a thin layer on the side of the ribbon which during the manufacture of the ribbon did not contact the cooling surface (free side).